In powered vehicles, a transmission receives rotational speed and torque output by the vehicle's drive unit (e.g. engine) and adapts it to the vehicle's operating requirements. The output of the transmission drives the vehicle load (e.g. the drive wheels).
Some vehicles have an auxiliary gearbox (e.g. a transfer case or dropbox), which is coupled to the transmission output. The auxiliary gearbox may be used to provide additional mechanical ratio coverage, an overdrive, and/or alternate modes of operation (e.g. four-wheel drive or all-wheel drive). If the vehicle is equipped with a power take-off assembly (PTO), the auxiliary gearbox may be used to shift the vehicle into a mode in which an auxiliary device attached to the PTO can be operated. In such event, the auxiliary gearbox drives the operation of the auxiliary device. The auxiliary device may be, for example, a pump, mixer, lifter, shoveler, compressor, compactor, or blower, as may be provided with utility trucks such as fire trucks, tow trucks, dump trucks, cement mixers, and the like.
The vehicle operator initiates use or shifting of the auxiliary gearbox, using a mechanical, pneumatic, hydraulic, or electrical selector to select the desired alternate mode of operation. However, often, the alternate mode cannot be used unless the vehicle is stopped and/or the transmission is in a neutral range.